The present invention relates to an electron spectrometer or energy filter which selects electrons of specific energy and forms an image and, more particularly, to an energy filter for a transmission electron microscope and an electron microscope provided with the energy filter.
In recent years, in the field of electrical, magnetic and structural materials as well as semiconductor devices, a fine structure in a material has been related to electrical, magnetic and mechanical properties of the material. Therefore, the technique of measuring and evaluating at a high spatial resolution is important as a basic technique for material development. A transmission electron microscope (TEM) is one of the measuring techniques having a high spatial resolution. Further, in order to not only observe micro structures but obtain knowledge of elemental analysis, chemical bonding analysis, etc., an electron energy loss spectroscopy (EELS) which analyzes the energy of electrons transmitted through the specimen is used (R. F. Egerton: Electron Energy-Loss Spectroscopy in the Electron Microscope. Plenum Press (1986)). Among the electron energy loss spectroscopys, a method in which an imaging energy filter is provided and the transmission electrons are two-dimensionally dispersed of energy is called an energy filtering transmission electron microscopy (EF-TEM (L. Reimer Ed: Energy-Filtering Transmission Electron Microscopy, Springer (1995))), and it is effective as a material evaluation method from the following two viewpoints:
(1) By separating inelastic scattering electrons which were background, an image and diffraction pattern of only zero loss electrons is obtained and quantitative evaluation of strength becomes possible (J. C. H. Spence and J. M. Zuo: Electron Microdiffraction, Plenum Press (1992)). PA0 (2) By observing the inelastic scattering electrons, it is possible to two-dimensionally detect information such as elemental analysis and chemical analysis. PA0 (1) Since the energy filter according to the present invention can be incorporated in any electron microscope, the restriction on expansibility which was a problem in the conventional in-column type is removed. PA0 (2) Since the entire height of the electron microscope is not made higher by installing the energy filter under the viewing chamber, the problems accompanied by making the instrument large in scale are solved with the in-column type energy filter. Further, a general electron microscope has a space the height of which is typically 60 cm or more under the camera chamber (or the viewing chamber). Such a space allows an energy filter of sufficient performance (for example, energy dispersion of 1 .mu.m/electron volt) to be provided for an electron microscope of 400 kv class or less which is used in general. PA0 (3) With the instrument provided with the in-column type energy filter, an operator who does not need energy filter can utilize the function of the electron microscope without changing the function, without operating the energy filter. Further, since the condenser aperture and the position of the specimen holder are not changed, the above-mentioned problem of reduction in operability is solved. PA0 (1) Since an aberration correction using complicated multipolar lenses is not needed, complicated axis adjustment is unnecessary and the operability is improved greatly. For example, in the conventional instrument, at least six factors (an achromatic image, x focus, y focus, spectrum distortion in an x direction, spectrum distortion in a y direction, and a ratio of length and breadth of an image) were required for the adjustment, however, the factors are reduced to 2 or so factors (an achromatic image, x focus). PA0 (2) Since the energy filter itself has a function to cancel the aberration, distortion of the image becomes small on principle. PA0 (3) The construction of the instrument becomes simple and the instrument can be produced at a low cost.
The energy-filteing transmission electron microscope can be classified as a type (in-column type) in which an energy filter is incorporated within the column of the electron microscope and as a type (post-column type) in which the energy filter is added to the rear portion of the microscope column.
An example of the in-column type is a transmission electron microscope disclosed in JP B 6-42358. As for the post-column type energy filter, a system is known which is disclosed in the papers by O. L. Krivanek, et al (O. L. Krivanek, A. J. Gubbens and N. Dellby: Microsc. Microanal. Microstruct. 2 (1991), 315.), for example. Features of the conventional technique are described hereunder regarding both the in-column type and the post-column type.
(a) A Conventional In-column Type Energy Filter
FIG. 5 is a schematic diagram of an electron microscope provided with a conventional in-column type energy filter. The electron microscope has a construction in which the energy filter is positioned between an intermediate lens system and a projection lens system.
An electron beam emitted from an electron gun is first accelerated by an accelerating voltage device. The electrons are converged by a condenser lens system, and fall on the specimen held by the specimen holder. The electron beam causes various interaction with the specimen, and a part of the electron beam loses partially its energy. An energy loss value depends on the interaction between the specimen and the electrons. The electron beam, having been transmitted through the specimen, is magnified by an objective lens and an intermediate lens system, and arrives at the in-column type energy filter. In this case, a final crossover of the intermediate lens system is formed on the crossover plane of the energy filter, and an image (for example, electron microscope image and diffraction pattern) by the electron beam that is desired to be filtered is formed on the incident image plane. Inside the in-column type energy filter, different tracks are generated according to the energy. In FIG. 5, an average track of electrons is expressed by a curved line. Since the average track draws a Greek letter .OMEGA., the energy filter is also called an .OMEGA. filter. As for the in-column type energy filter generating an electron track of the .OMEGA. type, JP A 62-66552 is known, for example. The electron beam from the crossover is converged at different positions on the energy dispersion plane depending on an energy loss value. Accordingly, on the energy dispersion plane, a spectrum corresponding to a quantity of electron energy loss, that is, an electron energy-loss spectrum is formed. Further, the electron beam from the incident image plane forms again an image on an achromatic image plane. Since an electron beam of different energy also forms an image at the same position (there is no energy dispersion), the image plane is called an achromatic image plane. The electron beam, having passed through the in-column type energy filter, is magnified by the projection lens system and projected on a fluorescent screen, an image recording device, etc., provided in a viewing chamber. The projection lens system and an energy slit have the following two functions according to objects:
One of the functions is to observe an energy filtered image by selecting electrons of interest energy and projecting the achromatic image plane on the fluorescent screen (or the image recording device). The other function is to measure an electron energy-loss spectrum by projecting electrons created on the energy dispersion plane onto the fluorescent screen (or the image recording device) without using the energy selection slit.
The .OMEGA. type track of electron beams within the in-column type energy filter can be achieved by using a plurality of electron spectrometers combined with each other. In this prior art, four magnetic sectors are used. The magnetic sector is an object to disperse electrons by utilizing the principle that the rotation radius of electrons in the magnetic field depends on the energy of electron.
The major feature of the conventional in-column type energy filter is in that the tracks of electron beams inside the energy filter are symmetric with respect to a symmetric plane. It is known that distortion of the image on the achromatic image plane is reduced owing to the symmetry of the track of electron beam, and this is an electron optics advantage of the in-column type. Further, a common feature to the conventional in-column type energy filter is in that a normal line to the symmetric plane and the direction of electrons incident on the energy filter are parallel to each other. This is for causing the direction of electrons incident on the energy filter and the direction of exit electrons to be the same. Of the in-column type energy filters, in addition to the energy filter generating the .OMEGA. type track shown in this figure, there are an energy filter in which an average track drawn by electrons is a .alpha. type (JP A 62-69456, JP A 7-37536, JP A 8-3699), a Castaing-Henry type energy filter in which an electrostatic mirror and a magnetic sector are combined, etc., however, all of them have the feature that an average track of electron beams is symmetric with respect to the symmetric plane and the normal line to the symmetric plane is parallel with the direction of incidence of the electron beams.
(b) Conventional Post-column Type Energy Filter
FIG. 6 is a schematic diagram of an electron microscope provided with a conventional post-column type energy filter. Optical components of the electron microscope from an electron gun to an image recording means are already known as a transmission electron microscope. A portion added to the rear portion of the electron microscope is a post-column type energy filter.
By retracting a fluorescent screen and the image recording device out of an optical axis of electron beams, the electron beams enter the post-column type energy filter. In the post-column type energy filter, energy dispersion is caused by a single magnetic sector. In this case, electrons from the crossover plane formed of a projection lens system disperse energy and are projected on an energy dispersion plane. The electron beams selected by an energy slit are magnified by multipole lenses and projected on an image detector. Those multipole lenses effects a similar operation to that of the projection lens system in the electron microscope with an in-column type energy filter, projects an image by the electrons selected by the energy slit onto the image detector and projects an energy loss spectrum on the energy dispersion plane onto the image detector.
The major feature of the conventional post-column type electron microscope is in that there is only one magnetic sector. Therefore, an achromatic image plane, which existed in the in-column type energy filter, is not formed by the single magnetic sector. Accordingly, it is necessary to form an achromatic image with the multipole lenses, etc. Further, it is already known that since an electron track has no symmetry, image distortions by the single magnetic sector are large (N. Ajika, H. Hashimoto, K. Yamaguchi and H. Endo: Japanese Journal of Applied Physics, 24 (1985), L 41). The multipole lenses requires the operations such as formation of an achromatic image and correction of distortion in image, so that the structure becomes more complex than the projection lens system in the in-column type energy filter system. In the above described papers (O. L. Krivanek et al (1991)), multipole lenses are 12 stages lenses comprising six stages of quadrupole and six stages of sextupole and are very complex.